The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

Civil Twilight over Salt Lake, Bahariya Oasis

The photo above shows an eye-catching twilight sky over Salt Lake, Bahariya Oasis, Egypt. Salt Lake is one of several salt lakes scattered in Egypt's Western Desert. Since it is also an interesting birding site I was in there to watch migrating birds as well as to observe the annual Geminid Meteor Shower. The Sun had just sunk below the horizon and while darkness began to descend mid- and high-altitude clouds in the western sky lit up when struck by flicks of sunlight. It was now civil twilight. Evening civil twilight begins after the Sun sets and lasts until the geometric center of the sun reaches six degrees below the horizon. During evening civil twilight, only the brightest stars and planets are visible, and the horizon can still be clearly defined. The tangerine and burnt orange colors of the sky are a result of the increased path length of sunlight when the Sun is positioned close to or just below the horizon. The shorter wavelength colors (indigo, blue and green) are scattered from view. Note also the lovely reflection on Salt Lake. Photo taken on December 11, 2008.